The function of a servo system for longitudinal tape, such as magnetic tape, is to move a head laterally of the longitudinal tape to accurately follow the lateral movement of the tape, for example, during read/write operations of the head. If done accurately, the data tracks are written and read by a read/write head in straight lines along the longitudinal tape as the tape is driven in the longitudinal direction. With respect to magnetic tape, the data comprises parallel stripes in the longitudinal direction of the magnetic tape. Servo tracks are prerecorded in the magnetic tape parallel to, and laterally offset from, the expected data stripes. The servo tracks are typically provided at separate lateral locations across the magnetic tape so that the magnetic tape may comprise many servo tracks and many sets of data bands. Servo heads, typically at opposite ends of a head module of the read/write head, sense two servo tracks and control the lateral positioning of the read/write head to track follow the servo tracks. The read/write head is shifted laterally across a set of servo tracks to shift between data tracks of a data band, and is shifted from one set of servo tracks to another to shift between data bands.
Embodiments of servo tracks may comprise longitudinal tracks arranged in sequential repeating patterns, one example comprising timing based servo tracks, and the patterns of the respective servo tracks are longitudinally offset from each other to allow a determination of the gross lateral position of the servo heads and the data band at which the magnetic head is positioned by measuring the longitudinal offset between the two servo tracks. Once the longitudinal offset of the servo tracks as sensed by the servo heads at opposite ends of the head module are known and therefore the data band is known, the relative measurement of the timing of the servo tracks as compared with the known longitudinal offset provides a measurement of the skew of the tape with respect to the read/write head, which can be compensated for, for example, by tilting the read/write head.
Typically, the lateral movement of the tape is constrained by flanges present on tape guides at either side of the head, such that the servo system causes the head to follow the data stripes in the presence of disturbances mainly created from limited lateral motion of the tape, called LTM (Lateral Tape Motion).
The flanges of the tape path, such as rollers, limit the lateral motion of the tape, but may tend to flex the tape and to introduce debris accumulation in the flanges that impact the lifetime of the tape and in addition create undesirable dynamic effects.
A flangeless tape path tends to solve the problems of the flanged tape path, but, without being constrained, the longitudinal tape tends to rapidly shift from one side of a tape path to the other, and it may run at one side of the path for only a short period.
The shift at a tape path at one side of a read/write head may be to the opposite side of the path which is at the other side of the read/write head, introducing substantial skew of the longitudinal tape.
Skew may be determined by measuring the longitudinal offset between servo tracks sensed by servo heads at the upper and lower ends of the tape head module. However, the servo tracks may be longitudinally offset at the manufacture of the tape to indicate the data band at which the read/write head is positioned. The skew due to a flangeless tape path may be such that it offsets or enhances the manufactured longitudinal offset of the servo tracks, inhibiting the ability to determine the gross lateral positioning (which servo tracks) of the read/write head. Incorrect or unknown gross lateral positioning, in turn, inhibits the ability to employ the use of the servo heads to measure the skew of the tape with respect to the read/write heads, since the zero skew position of the servo tracks is unknown.
The servo skew actuator may have built in biases that cause the read/write head to be in a non-ideal position relative to the servo tracks as a tape is initially moved with respect to the read/write head, for example, when a magnetic tape cartridge is loaded into a data storage drive and the tape is moved across the read/write head. These biases can be caused by the effects of flex cable bias, from biases caused by gravity on the actuator mechanism, from external forces of shock and vibration, and from tape motion forces on the read-write head.